Sludge is the solid end product of sanitation plants. Its disposal has created problems for county and municipal governments for a long time, and these problems are becoming more severe as concern for the environment increases. Considerable effort has been expended in attempts to solve these problems, and this effort is reflected in the patent art.
Among the patents that have been granted in this area are the following U.S. patents:
U.S. Pat. No. 3,981,800 is concerned with the production of high quality of methane gas. In this process, dry manure is blended with water and seed sludge to the desired consistency and then the mixture is subjected to anaerobic digestion for a period of days in a digester. The mixture then passes into a second digester where it again is subjected to anaerobic digestion for several more days. Methane gas is recovered from each digester while some sludge is recirculated from the second digester to the first digester. Remaining sludge is thickened, partially recirculated to the blender and partially dewatered. The dewatered sludge is dried, using methane as the heat source, to form a dry marketable product, which is presumably a fertilizer.
U.S. Pat. No. 4,040,953 is directed to a process for conversion of organic material to methane gas and a residue suitable for use as a soil conditioner, organic fertilizer, or protein-rich animal feed supplement. An organic slurry, from which grit, inorganic solids and carbon dioxide have been stripped, is passed through a multi-stage anaerobic digestion for the production of methane gas. After removal of methane, the sludge is partially recycled to the carbon dioxide stripping step. The remainder of the sludge is thickened and dewatered to form a solid residue.
U.S. Pat. No. 4,267,049 converts treated sludge from raw municipal waste waters or raw agricultural wastes into organic feedstock or fertilizer. The treated sludge is passed through a hydrolytic enzyme conditioning and then tissue and cells are mechanically disintegrated. Heavy metals are removed, and the sludge is subjected to autolysis by an infusion of fresh endocellular enzymes. The autolysate product is dewatered, and the resultant solid may be used as an organic feedstock or as high quality fertilizer.
U.S. Pat. No. 4,369,194 defines a process in which manure is finely ground and then mixed with water to produce a mixture having at most 4% solids. Filaments and vegetable fibers are removed in a separator, and the liquid is fed to a bioreactor for the production of methane. After a suitable time in the digester, e.g., four days, and removal of methane, the resultant suspension is fed into a separator for removal of al solid substances. The solid substances are then dried, using methane produced in the digester as a heat source.
U.S. Pat. No. 4,388,186 concentrates waste water from a solids content of about 1% to a solids contents of about 6% to about 10% in a centrifugal condensing machine. This concentrate is then treated in an anaerobic digestion tank. The digested sludge is then dewatered and dried. Methane, which was produced in the digester, provides heat to the drier.
U.S. Pat. No. 4,632,758 utilizes a honeycombed reactor to generate methane and sludge from waste water as derived from a food processing plant. The waste water is first freed of debris such as cans, boards, and large vegetable scraps. It is then heated prior to introduction into the reactor, in which organic matter is converted into methane and sludge in an anaerobic process. Methane thus produced is used to heat the waste water prior to its introduction into the reactor; excess methane is burned off. Some sludge is recirculated to different parts of the system and the rest is withdrawn from agricultural utilization without further treatment.